Redox-reactive minerals are naturally occurring solid materials that are partly or fully-composed of redox-reactive elements (Gilbert & Waychunas, this volume p. 55). In chemistry, a redox reaction is the sum of two reactions which involve electron exchange: a reduction reaction and an oxidation reaction. So redox-reactive minerals are naturally occurring solid chemical compounds which are involved in electron exchange reactions. This might include most minerals since although many cations do not display variable oxidation states in the natural environment, most anions found in minerals, such as oxygen and sulfur, do. However, in the context of this volume, redox-reactive minerals are...